In a conventional controlled potential electrolysis gas sensor, its electrodes are disposed to face the interior of an electrolysis solution accommodating portion of an electrolysis solution tank that holds electrolysis solution in a sealing manner; and as the electrodes, there are provided e.g. three electrodes consisting of a working electrode for electrochemically reacting gas to be detected, a counter electrode with respect to the working electrode, and a reference electrode for controlling potential of the working electrode. And, there are connected the electrolysis tank holding electrolysis solution to which the electrodes can contact, a potentiostat circuit or the like for setting potentials of the respective electrodes. As materials for forming the three electrodes above, a noble metal catalysis of platinum, gold or palladium or the like is applied to a gas-permeable, porous PTFE membrane having water repellant property was employed and as electrolysis solution, acidic water solution of sulfuric acid, phosphoric acid or the like was employed.
Further, the controlled potential electrolysis gas sensor is configured to generate, between the working electrode and the counter electrode, a current corresponding to an environmental change in the surrounding by controlling potential of the working electrode relative to such environmental change in the surrounding to maintain it constant. Further, with utilization of the potential of the working electrode being maintained constant and an oxidization reduction potential varying according to gas species, according to potential set by the potentiostat circuit, selective detection of gas is made possible. Also, by changing the catalyst to be used for the gas electrode, it is possible to provide high selectivity for a target gas.
As a noble gas catalyst to be applied to an electrode, it was known to employ one having gold particles in the order of several hundreds nm supported to carbon having a particle size of few tens of nm, for instance. In order to cause carbon to support gold particles as described above, the immersion supporting technique is sometimes employed. When noble metal particles are to be supported to a support by this immersion supporting technique, this support is immersed in a water solution of metal salt for allowing metal component to be adsorbed to the surface of the support. Then, drying, sintering and reduction are carried out. After fabrication of gold supported carbon by the above immersion supporting technique, this would be applied to a porous PTFE membrane, thus forming an electrode.
Incidentally, the above-described controlled potential electrolysis gas sensor constituting a conventional art for the present invention is a standard technique, so no particular prior-art documents relevant thereto such as patent documents will be cited herein.